It is known that thermal power plants have a water-steam circuit (thermal circuit) in which a feedwater tank is arranged among other things. The feedwater tank is used for storing and reconditioning the water that is circulating in the thermal circuit. The degasification of the water is particularly important during the reconditioning process.
In all thermal power plants a most complete degasification possible of the feedwater is indispensable for reliable and safe operation and also for achieving the highest efficiency of the plant. The gases to be removed include particularly the gases that are not condensable at the temperature prevailing in the feedwater tank such as for example oxygen and carbon dioxide. Furthermore, especially in a nuclear power plant of the boiling-water reactor type, hydrogen and oxygen, also called photolysis gases, are formed by the effect of the radioactive radiation on the feedwater that is circulating through the reactor pressure vessel.
The oxidizing properties of the oxygen lead to corrosion and thus damages to all the components of the thermal circuit. These damages drastically reduce the lifespan of the components. Moreover, the hydrogen gas that results from photolysis presents an enormous safety risk, because it is highly flammable and forms particularly with the existing oxygen gas a highly explosive mixture, the so-called detonating gas. However in order to ensure a safe operation of the boiling-water reactor, the resulting photolysis gas must be removed as completely as possible from the feedwater.
One alternative for removing hydrogen and oxygen from the water contained in a primary circuit of a nuclear power plant is the conversion of both the gases by means of catalytically active surfaces according to the patent application DE 3614267 A1.
In another process of the so-called spray degasification, the water containing the gases is sprayed above the feedwater present in the feedwater tank in the form of the finest water droplets and brought in contact with hot steam, the so-called extraction steam. The heat exchange of the extraction steam with the sprayed colder water causes the gas dissolved in it to be released and removed from the thermal circuit by a suitable suction device. The spring-loaded water spray valve known from DE 10001297 can be used for spraying the water in the feedwater tank.
However the suction of the gases released by the spray degasification presents some fundamental problems. It has been seen in practice that the spray shield formed by the spraying of the water forms a relatively dense cover that interferes with the suction of the gases by the suction device arranged in the feedwater tank. An insufficient suction of the radiolysis gases leads to the afore-mentioned disadvantages and risks in the operation of the thermal circuit.
Also the even distribution of the water to be degasified by the spraying device and the even distribution of the extraction steam in the feedwater tank is considerably difficult due to the incomplete suction of the radiolysis gases. Thus the optimal efficiency of the spray degasification and ultimately of the entire thermal circuit cannot be achieved.